US5867012A

US5867012A - Switching bandgap reference circuit with compounded ΔV.sub.βΕ

Info

Publication number: US5867012A
Application number: US08/907,839
Authority: US
Inventors: Michael G. Tuthill
Original assignee: Analog Devices Inc
Current assignee: Analog Devices Inc
Priority date: 1997-08-14
Filing date: 1997-08-14
Publication date: 1999-02-02
Anticipated expiration: 2017-08-14

Abstract

A switching bandgap reference circuit with compounded ΔV_BE includes an amplifier having an output, an inverting input and a non-inverting input; a first PN junction connected to the non-inverting input; a second PN junction connected to the inverting input through an input capacitor; a low current source and a high current source; a switching device for applying in the auto zero mode the low current source to a first terminal of the first PN junction and the high current source to a first terminal of the second PN junction for establishing the V_BE, of the first junction at both the inputs of the amplifier and for applying in the valid reference mode the high current source to the first terminal of the first PN junction and the low current source to the first terminal of the second PN junction for establishing the positive ΔV_BE, of the first PN junction to both the inputs of the amplifier and applying the negative ΔV_BE2 of the second PN junction to the input capacitor to produce a voltage of ΔV_BE1 plus (-ΔV_BE2) across the input capacitor; a feedback capacitor connected between the output and inverting input of the amplifier to define the gain on the combined ΔV_BE voltages to produce a temperature stabilized voltage at the output of the amplifier and a reset switching device for discharging the feedback capacitor and enabling the amplifier to equalize its inputs in the auto zero mode.

Description

FIELD OF INVENTION

This invention relates to a switching bandgap reference circuit with compounded ΔV_BE.

BACKGROUND OF INVENTION

Bandgap reference circuits are used to provide a stable reference voltage independent of temperature variation. One continuous time bandgap reference circuit known as a Brokaw cell utilizes two matched bipolar transistors operating at different current densities to develop V_BE which decreases with temperature and ΔV_BE which increases with temperature and scales the ΔV_BE with a resistor network to just offset the V_BE decrease to produce the stabilized reference voltage. Another continuous time bandgap reference circuit fabricated in CMOS technology makes use of the parasitic substrate bipolar transistors to combine V_BE and the offseting ΔV_BE to obtain the temperature stabilized reference output voltage. In the construction the transistors must be matched and the output is a high impedance instead of the operational amplifier of the Brokaw cell. In this construction and in the Brokaw cell the continuous nature of the output makes it difficult to correct the inherent amplifier offset.

In another approach, as disclosed in U.S. Pat. No. 5,563,504, Gilbert et al., a single PN junction is used to develop both the V_BE and offsetting ΔV_BE but it is a switching band gap reference circuit which alternates between two modes: the auto-zero mode in which the output voltage is not temperature stabilized and the valid reference mode in which it is. The PN junction is typically a parasitic substrate bipolar transistor in a CMOS circuit. In these circuits the ΔV_BE is relatively quite small which does not help with signal to noise ratio considerations. One way to increase ΔV_BE is to increase the current density ratio but this only aids up to a point due to the logarithmic nature of ΔV_BE with respect to current density. Further, when the current density is increased the ability to match the PN junction becomes more difficult.

SUMMARY OF INVENTION

It is therefore an object of this invention to provide an improved switching bandgap reference circuit.

It is a further object of this invention to provide such a switching bandgap reference circuit which increases the ΔV_BE voltage.

It is a further object of this invention to provide such a switching bandgap reference circuit which has an improved signal to noise ratio.

It is a further object of this invention to provide such a switching bandgap reference circuit which increases ΔV_BE independent of the current density ratio of the PN junction.

It is a further object of this invention to provide such a switching bandgap reference circuit in which the PN junctions need not be matched.

It is a further object of this invention to provide such a switching bandgap reference circuit in which the amplifier offset error and low frequency noise are easily corrected.

The invention results from the realization that an improved switching bandgap reference circuit with increased ΔV_BE can be effected without increasing the current density ratio by developing ΔV_BE voltage across a plurality of PN junctions and summing those ΔV_BE voltages to produce a larger total ΔV_BE voltage at the input to the amplifier to be scaled to the temperature stabilized output voltage by the feedback network of the amplifier.

This invention features a switching bandgap reference circuit with compounded ΔV_BE. There is an amplifier having an output, an inverting input and a non-inverting input, a first PN junction connected to the non-inverting input, and a second PN junction connected to the inverting input through an input capacitor. There is a low current source and a high current source. A switching device applies in the auto zero mode the low current source to a first terminal of the first PN junction and the high current source to a first terminal of the second PN junction for establishing the V_BE, of the first junction at both the inputs of the amplifier and for applying in the valid reference mode the high current source to the first terminal of the first PN junction and the low current source to the first terminal of the second PN junction for establishing the positive ΔV_BE1 of the first PN junction to both the inputs of the amplifier and applying the negative ΔV_BE2 of the second PN junction to the input capacitor to produce a voltage of ΔV_BE1 plus (-ΔV_BE2) across the input capacitor. A feedback capacitor is connected between the output and inverting input of the amplifier to define the gain on the combined ΔV_BE voltages to produce a temperature stabilized voltage at the output of the amplifier. A reset switching device discharges the feedback capacitor and enables the amplifier to equalize its inputs in the auto zero mode.

In a preferred embodiment the amplifier may be an operational amplifier, the PN junction may be included in the diode or a transistor, and the transistor may be a bipolar transistor. The PN junction may be included in a parasitic substrate bipolar transistor in a CMOS circuit. The first PN junction may have second terminal connected to a common node to establish the amplifier output voltage relative to that node. The collectors of the transistors may be connected to ground.

DISCLOSURE OF PREFERRED EMBODIMENT

Other objects, features and advantages will occur to those skilled in the art from the following description of a preferred embodiment and the accompanying drawings, in which:

FIG. 1 is a schematic diagram of a prior art Brokaw cell bandgap reference circuit;

FIG. 2 is a schematic diagram of a prior art continuous time CMOS bandgap reference circuit;

FIG. 3 is a schematic diagram of a prior art switching bandgap reference circuit;

FIG. 3A illustrates waveforms that occur in the circuit of FIG. 3;

FIG. 4 is a schematic diagram of a switching bandgap reference circuit with compounded ΔV_BE according to this invention;

FIG. 4A illustrates waveforms that occur in the circuit of FIG. 4; and

FIG. 5 is a schematic diagram of the circuit of FIG. 4 with amplifier offset cancellation.

A stable reference voltage is required in many electronic systems and integrated circuits, especially in analog to digital converters where an input voltage is to be measured with respect to a stable voltage and in digital to analog converters where the output voltage is a code-dependent fraction of this reference voltage. This voltage must be generated on-chip and should be independent of temperature.

One well-known technique for generating such a reference voltage is the so-called bandgap reference. This type of circuit utilizes the fact that the base-emitter voltage (V_BE) of a bipolar transistor decreases with increasing temperature in a well-behaved manner. This behavior is known as CTAT or Complementary-to-Absolute Temperature. If a voltage which is PTAT, or Proportional-to-Absolute-Temperature, is generated and added with the correct scaling to the V_BE of the bipolar transistor, then the resulting voltage may be made independent of temperature.

FIG. 1 shows one prior art example known as the Brokaw cell 10 to illustrate this technique. In this example,

transistors

12 and 14 are operated at equal currents I₁, I₂ but are of unequal emitter area giving rise to a difference in Base-Emitter voltage which may be written as:

ΔV.sub.BE =(kT/q)·1n(N)

and is clearly PTAT as it contains the term T, absolute temperature. N is the current density ratio or in this case the area ratio since the currents are equal. This equality is established by making resistance 16 equal to resistance 18. A value of N=8 is shown which may be achieved by scaling emitter area or, more accurately, by use of multiple devices. This ΔV_BE appears across resistance 20 and determines the operating current of transistor 12 and thereby transistor 14. The sum of these currents flows in resistance 22, developing a voltage which is added to the V_BE of transistor 14 and appears at the amplifier 24 output. Thus by proper choice of N, resistance 20 and resistance 22, the output voltage may be chosen such as to be independent of temperature as previously discussed. For more detail see A. Paul Brokaw, "A Simple Three Terminal Bandgap Reference", IEEE J. Solid-State Circuits, Vol. SC9, pp. 288-393, Dec. 1994.

Two problems exist with this circuit, firstly isolated bipolar transistors are required and so the circuit cannot be implemented in a low cost CMOS process and secondly good V_BE matching is required between two transistors operating at different current densities.

In a standard CMOS process one bipolar device type is available. This is the parasitic substrate device which has its collector at the common wafer potential. In an N-well CMOS process this potential is ground in a single supply design. FIG. 2 shows a continuous time circuit 30 using these

substrate PNP transistors

32, 34 with the same scaling techniques of FIG. 1 used to produce a ΔV_BE across resistance 36 and so establish the operating current of the circuit. The amplifier 38 sets up the gate-source bias of transistors 40 and 42 so as to provide this operating current. A further copy of this current is made by transistor 44. This develops a voltage drop across resistance 46 which is added to the V_BE of transistor 48 so as to produce a stable bandgap reference voltage. This circuit still has the problem of matching between

transistors

32 and 34 and also suffers from poor offset voltage in the CMOS input pair of the amplifier 38. Circuits of this type have been used to make bandgap references on CMOS processes but they lack precision.

One further prior art example is shown in FIG. 3, described in U.S. Pat. No. 5,563,504, issued to Barrie Gilbert and Shao-Feng Shu, "Switching Bandgap Voltage Reference". This utilizes the fact that in circuits such as A/D converters, the reference voltage is not required to be stable at all times and so may be switched from an auto-zero mode to a valid reference mode and back again as required. This enables the use of only one bipolar transistor operated at different switched currents and so eliminates the problem of mismatch. During the auto-zero mode switch 50 is closed and 52 is open. This sets up the initial conditions of the circuit with V_out =V_BE1 at I₁. In the valid reference mode, switch 50 opens first, storing V_BE1 on capacitor 52. Then switch 52 closes and the emitter 54 of transistor 56 increases to V_BE1 at I₁ +I₂. This change in voltage is amplified by a gain determined by the feedback capacitances 52 and 58 and added to the output such that: ##EQU1## This eliminates the need for good matching between the two

bipolar transistors

12 and 14 used in FIG. 1 and

transistors

32 and 34 used in FIG. 2 but there remains a problem with amplifier offset and noise as the ΔV_BE signal is small. For example with the area ratio 8:1 of

transistors

12 and 14, FIG. 1, the ΔV_BE signal at room temperature is only 53 mV approximately. This signal can be increased by using a larger ratio in the currents I₁ and I₂ but with an increase in power dissipation. Also, the effect is not dramatic due to the log term. For example with N=80 rather than 8, ΔV_BE =112 mV. The development of V_in and V_out in relation to the clock signal CLK1 and CLK2 that operate switches 50 and 52 are shown in FIGS. 3A.

The new switched current bandgap reference circuit 80 with compounded ΔV_BE is shown in FIG. 4. The purpose of this circuit is to generate a 2×ΔV_BE signal from one switching action of the

current sources

62, 64. Two

PNP transistors

66, 68 are now used with currents I and N(I) and the current is switched from one to the other so that as one V_BE goes from a low to a high value, the other V_BE goes from a high to a low value maintaining a constant overall supply current. The resulting two ΔV_BE signals are summed on capacitor 70 and amplified by amplifier 72 as before to produce the required bandgap voltage at the output of amplifier 72. Although two

PNP transistors

66, 68 are now used there is no matching requirement between them. Only the ΔV_BE of each transistor at the switched currents is summed with the V_BE of transistor 66.

For description purposes the various V_BE 's are designated as follows: V_BE1(1) and V_BE1(NI), are the V_BE of transistor 66 at I and N(I) and V_BE2(1) and V_BE2(N1) are the V_BE of transistor 68 at I and NI. Initially the circuit is in auto-zero with the feedback switch 74 closed, shorting feedback capacitor 75, and with

switches

76 and 78 set to the left. Thus the output is at V_BE1(1), FIG. 4A, as the amplifier 72 is configured as a voltage follower. The right hand side of capacitor 70 is also at V_BE1(1). The left-hand side of capacitor 70 is at V_BE2(N1). Switch 74 is now opened, preserving these initial conditions.

Switches

76 and 78 are now switched to the right, causing the voltage at the positive terminal of the amplifier 72 to increase by an amount V_BE1(N1) -V_BEI(1) !. If the left-hand terminal of capacitor 70 were at a fixed voltage then the output would respond to this signal only and the circuit would default to that of FIG. 3. However, as the positive terminal and thereby the right-hand side of capacitor 70 increases by this ΔV_BE1, the current in transistor 68 reduces from NI to I causing a drop in voltage of V_BE2(N1) -V_BE2(1) ! on the left-hand side of capacitor 70. Thus both ΔV_BE terms are summed and amplified to give the following output: ##EQU2##

Under nominal conditions, both ΔV_BE signals are equal and the circuit 60 simply doubles this ΔV_BE input signal. This improvement is best illustrated with an example: With a unit current source I=2μA and N=24 giving a total switching supply current of 50μA, there is a ΔV_BE signal of 81.65 mV at T=25 degrees. The input signal to the amplifier is then 163.3 mV. To achieve the same input signal level with the circuit of FIG. 3 a much larger ratio would be required: 2μA and (24)² ×2μA giving a total switching current of 1152μA.

As the amplifier 72 is made using MOS input devices, a significant offset voltage must be expected and so offset cancellation circuitry should be included. FIG. 5 shows a well-known technique for offset cancellation sometimes known as correlated-double sampling or simply auto-zero. In auto-zero the feedback switch 74 is closed but the feedback capacitor 75 is no longer connected to the amplifier 72 output as before in FIG. 4. Instead, capacitor 75 is switched to the positive input of the amplifier 72 by switch 80 and so is charged to the amplifier offset voltage. When the circuit switches from auto-zero to the valid reference mode, switch 74 opens first, storing this offset on capacitor 75. Then capacitor 75 is switched back to the amplifier 72 output by switch 82, cancelling the offset error and finally switches 76 and 78 are switched to create the required ΔV_BE signals.

The ratioed current sources are made with a 5×5 array of unit PMOS devices which are laid out with the same care as used in making 8 and 10 bit current-source Digital to Analog converters so the matching performance may be predicted and is well within the requirements of the circuit. The array is surrounded by a full ring of dummy devices with the center PMOS device chosen as the single 2μA current source. The surrounding 24 devices are wired together to give 48μA.

Although specific features of this invention are shown in some drawings and not others, this is for convenience only as each feature may be combined with any or all of the other features in accordance with the invention.

Other embodiments will occur to those skilled in the art and are within the following claims:

Claims

What is claimed is:

1. A switching bandgap reference circuit with compounded ΔV_BE, comprising:

an amplifier having an output, an inverting input and a non-inverting input;

a first PN junction connected to said non-inverting input;

a second PN junction connected to said inverting input through an input capacitor;

a low current source and a high current source;

a switching device for applying in an auto zero mode the low current source to a first terminal of said first PN junction and the high current source to a first terminal of said second PN junction for establishing a V_BE1 of said first PN junction at both said inputs of said amplifier, and for applying in a valid reference mode the high current source to said first terminal of said first PN junction and the low current source to said first terminal of said second PN junction for establishing a positive ΔV_BE1 of said first PN junction to both said inputs of said amplifier and applying a negative ΔV_BE2 of said second PN junction to said input capacitor to produce a voltage of ΔV_BE1 plus (-ΔV_BE2) across said input capacitor;

a feedback capacitor connected between the output and inverting input of said amplifier to define the gain on a combined ΔV_BE voltage to produce a temperature stabilized voltage at the output of said amplifier; and

a reset switching device for discharging said feedback capacitor and enabling said amplifier to equalize its inputs in the auto-zero mode.

2. The switching bandgap reference circuit of claim 1 in which said amplifier is an operational amplifier.

3. The switching bandgap reference circuit of claim 1 in which at least one of said first or second PN junctions is included in a diode.

4. The switching bandgap reference circuit of claim 1 in which at least one of said first or second PN junctions is included in a transistor.

5. The switching bandgap reference circuit of claim 1 in which at least one of said first or second PN junctions is included in a bipolar transistor.

6. The switching bandgap reference circuit of claim 1 in which at least one of said first or second PN junctions is included in a parasitic substrate bipolar transistor in a CMOS circuit.

7. The switching bandgap reference circuit of claim 6 in which said transistors include a collector connected to ground.

8. The switching bandgap reference circuit of claim 1 in which said first PN junction has a second terminal connected to a common node to establish the amplifier output voltage relative to that node.

9. The switching bandgap reference circuit of claim 8 in which said common node is ground.